The present disclosure generally relates to acidizing subterranean formations, and, more specifically, to methods for acidizing subterranean formations in the presence of a chelating agent that is initially ineffective at complexing a metal ion.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, remediation operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. More specific examples of illustrative treatment operations can include drilling operations, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.
Acidizing operations may be performed to stimulate a subterranean formation to increase production of a hydrocarbon resource therefrom. Introduction of the acidizing fluid to the subterranean formation may take place at matrix flow rates above or below the fracture gradient of the matrix. During a matrix acidizing operation, an acid-soluble material in the subterranean formation is dissolved by one or more acids to expand flow pathways in the subterranean formation, to create new flow pathways in the subterranean formation, to remove acid-soluble precipitation damage in the subterranean formation, and/or to increase flow to/from the matrix. The acid-soluble material may be part of the native formation matrix. Illustrative acid-soluble materials native to the formation matrix that may be dissolved by an acid include, but are not limited to, carbonates, silicates, and aluminosilicates. Other substances can also be dissolved during the course of performing an acidizing operation, and the foregoing substances should not be considered to limit the scope of substances that may undergo acidization.
Carbonate formations contain minerals that comprise a carbonate anion (e.g., calcite (calcium carbonate) and dolomite (calcium magnesium carbonate)). When acidizing a carbonate or dolomite formation, the acidity of the treatment fluid is usually sufficient to solubilize the matrix to a carbonate anion and leaching the metal ion (Ca2+, Mg2+) into the treatment fluid. The carbonate anion will further decompose to the carbon dioxide and water. Both mineral acids (e.g., hydrochloric acid) and organic acids (e.g., acetic and formic acids) can be used in carbonate formations, often with similar degrees of success to achieve this dissolution profile.
Siliceous formations include minerals such as, for example, zeolites, clays, and feldspars. As used herein, the term “siliceous” refers to a substance having the characteristics of silica, including silicates and/or aluminosilicates. Dissolution of siliceous materials through acidization is thought to be considerably different than acidizing carbonate materials because many acids that effectively acidize carbonate materials have little effect on siliceous materials. In contrast, hydrofluoric acid, another mineral acid, can react very readily with siliceous materials to promote their dissolution.
In addition to siliceous materials, many types of siliceous formations can also contain varying amounts of carbonate materials. Most sandstone formations, for example, contain about 40% to about 98% sand quartz particles (i.e., silicas), bonded together by various amounts of cementing materials, which may be siliceous in nature (e.g., aluminosilicates or other silicates) or non-siliceous in nature (e.g., carbonates, such as calcite).
In some instances, it can be desirable to remove carbonate materials from a siliceous formation prior to acidizing the siliceous material therein. A leading reason to remove a carbonate material separately from a siliceous material is that calcium ions liberated from the carbonate material can react readily with fluoride ions of hydrofluoric acid to form highly insoluble calcium fluoride that reduce flow through the matrix.
Precipitation of formation cations due to acidizing carbonate can also be problematic. Although most carbonate mineral cations can be dissolved with strong acid treatment fluids, dissolution of the formation matrix spends the acid. As the pH of the treatment fluid rises, cations can precipitate and damage the formation.
The precipitates formed during matrix acidizing, whether from siliceous or carbonate portions of the formation, may reduce the increased permeability achieved during acidizing, which may reduce the hydrocarbon production capacity of the formation.